Experts say a broad coalition is needed to defeat threat on ground, but assembling one is tricky

It took French President François Hollande to flatly declare it: In the wake of the terror attacks in Paris, the struggle against Islamic State isn’t merely a fight. It is a war.

There’s no shortage of aggrieved or alarmed nations to engage in this war, nor of planes to do their part in waging it from the air. What is lacking is an actual army to fight it. “The fundamental problem,” says Richard Haass, president of the Council on Foreign Relations, “is we still don’t have a ground partner.”